(376aj) Molecular Simulation on Separation of CO2/CH4 Mixture By Carbon Membrane with Zigzag Pore Structure

Combustion of natural gas (CH₄) can reduce carbon dioxide (CO₂) emission by 40% compared with coals and crude oils used widely. CH₄ is therefore accepted as a relatively clean and environmentally friendly energy. Separation of CO₂/CH₄ mixture is an essential process in the natural gas industry. Membrane technique is one of the most effective options for gas separation. As a promising material, carbon molecular sieve membrane (CMSM) yields well-developed pore structures and high permselectivity. Compared with experimental study, modelling can provide reproduction and prediction of a process of interest, allowing ones to gain insight into the process on the microscopic scale and even to reveal some new information. However, classical transport theory could not be suitable for nano-scale pore structure like CMSM. Molecular simulation is an appropriate choice to express permeation and separation behaviours of molecules. Previous modelling of gas separation in CMSM assumed a uniform and defect-free pore structure. Recent experimental investigations showed that some pore imperfections like malposition and defect were actually existent inside CMSM. Such imperfections may be the reason why simulation results did not agree with experimental data. It is therefore important to develop a new pore structure to represent realistic pores in CMSM concisely.

Zigzag-type pore structure was proposed for the separation of CO₂/CH₄ binary mixture by CMSM. Defect-free pore and three kinds of defect pores, i.e., random, uniform and partial defects, were considered to improve the separation performance. Adsorption and diffusion behaviours of pure gases and gas mixture were simulated by the Grand-Canonical Monte-Carlo (GCMC) method and the Non-Equilibrium Molecular Dynamics (NEMD) method, respectively. Operating pressure ranged from 10 to 100 kPa and temperature was between 273 and 348 K during simulation. The simulative isotherms of CO₂ and CH₄ showed an acceptable agreement with the experimental data for the defect-free pore of 0.67 nm in size at 298 K. Examination on the effect of pore sizes showed that 0.67 nm was the appropriate option for separation. The total selectivity was 20.1 at 298 K and 100 kPa with the pore of 0.67 nm, which is consistent with the experimental value. The adsorption was determined as the dominant separation mechanism between adsorption and diffusion. Compared with the defect-free pores, the introduction to the random and uniform defect pores can improve the total selectivities and the random defect provided the superior separation performance. Appropriately low temperature and small pore size were beneficial to the separation.

Acknowledgement

The authors express their sincere appreciation to the financial supports from the Fundamental Research Funds for the Central Universities of China (DUT17JC07) and the National Natural Science Foundations of China (21376037, 21436009).

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